Insulin, a hormone required for metabolic homeostasis, is produced only in the pancreatic beta cell. Type I Diabetes is characterized by a rapid loss of beta cell mass and a sharp decrease in pancreatic insulin content. A decline in beta cell mass and function is also characteristic of Type II Diabetes. Intervention to improve beta cell mass and function is a major goal of diabetes therapeutics.
Perturbations in the expression or function of numerous islet transcription factors have been demonstrated to cause beta cell dysfunction. In humans, mutations in islet transcription factors HNF4alpha, HNF4beta, HNF1alpha, and pdx-1 lead to Maturity Onset Diabetes of the Young (MODY) syndromes via the resulting deficit in beta cell function. See, e.g., Froguel, P & Velho, G, Trends Endocrinol Metab 10: 142-146 (1999)). Functional deletion of other islet transcription factors (e.g. nkx6.1, nkx2.2, isl-1, neuroD/beta2, PAX6) in mice also lead to beta cell and insulin deficits and diabetes. See, e.g., Melloul, D. et al., Diabetes 45: 309-326 (2002); Sander M et al., Genes Dev 11: 1662-1673 (1997); Sander M et al., Development 127: 5533-5540 (2000). Overexpression of pdx1 in islets can lead to restoration of beta cell mass and function in the context of diabetogenic mutations in other proteins (Kushner J A et al., J Clin Invest 109: 1193-1201 (2002)). Overexpression of pdx-1 and isl-1in enteric stem cell populations confers on them the ability to produce insulin (Kojima H et al., Diabetes 51: 1398-1408 (2002)). It is clear that the proper complement of islet transcription factors is crucial for maintenance of a stable metabolic state, and that, in some cases, it is possible to confer on certain cells the ability to express insulin by providing for increased expression of islet transcription factors.
There are five known RFX genes in humans. RFX1, RFX2, RFX3 and RFX4 share conserved DNA-binding domains, dimerization domains, and domains of undefined function called B and C. The dimerization domains allow RFX1-4 proteins to homo- and heterodimerize. This dimerization function is involved in the transcriptional function of these proteins. RFX 1, 2 and 3 also have an additional conserved A domain. RFX5 has the conserved DBD, but lacks the dimerization and A, B, and C domains. RFX5 controls transcription from the MHC class II gene promoter, and mutations in RFX5 lead to the bare lymphocyte syndrome, a serious immunodeficiency disorder (Reith W and Mach B, Annu. Rev. Immunol. 19: 331-373 (2001)).
The molecular mechanisms that control a cell's ability to produce insulin remains poorly understood. The present invention addresses this and other problems.